The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Recently, an environmental problem has been on the rise around the globe, and thus a method of reducing fuel to cope with this problem encompassing all industries has been sought. In order to reduce fuel, examples of a solution proposed in a vehicle industrial field include improving efficiency of a vehicle engine and a weight reduction in vehicles. By reducing the weight of vehicles, this helps increase fuel efficiency of the vehicle. However, when reducing the weight of vehicles, there occurs a problem in that strength and durability required in vehicles are not satisfied. Therefore, it is the greatest goal of a vehicle industry to solve this.
Generally, a steering rack bar of the vehicle is a part of a device adjusting an angle of a shaft of the vehicle so that a progress direction of the vehicle is changed according to an operation of a driver. FIG. 1 is a perspective view of a steering gear box assembly and a rack bar. If a steering handle is rotated, rotation force is transferred through a steering main shaft of a steering column 100 to a universal joint 200, and rotation force transferred to the universal joint 200 may be transferred through a pinion gear and a rack gear in a gear box 300 to a wheel of the vehicle to change the progress direction of the vehicle.
The rack gear is connected to a rack bar 400. In addition, the rack bar 400 receives rotation force from the pinion gear. The rack bar 400 corresponds to a device changing a steering angle of the wheel of the vehicle and thus changes the angle of the wheel of the vehicle so that the driving course of a vehicle is changed.
As described above, since the steering rack bar receives a load of the vehicle, a material for the steering rack bar needs to have high strength and a property enduring pulling force, that is, toughness which is sufficiently high. In addition, in the case where the vehicle runs on a road, if the steering rack bar is broken, there occurs a large problem in a safety risk to a driver, and thus the material of the steering rack bar needs to have high strength and sufficient impact strength. Further, in the case where the steering rack bar is manufactured, since a carbon steel composition needs to be subjected to cutting processing, a property of easily performing such processing is also desired.
In order to satisfy the aforementioned requirement, in the related art, two solutions are proposed. A first solution is to develop a high strength material. In addition, a second solution is a method of increasing a diameter of the steering rack bar.
In the related art, in a method of developing the high strength material, the high strength material developed in the related art has problems in that due to high strengthening, impact strength and processability are reduced and a thermal strain occurs.
In the related art, a method of increasing the diameter of the steering rack bar is used to improve strength, toughness, and impact strength of the steering rack bar. However, if the diameter of the steering rack bar is increased and thus a volume of the rack bar is increased, there is a design limitation of parts due to interference with peripheral parts. Further, there are other problems. If a weight of the steering rack bar is increased, a steering quality of the vehicle is reduced and fuel efficiency is reduced.
In addition to this, recently, in accordance with appearance of a technology such as R-MDPS (motor driven power steering R type), a high strength material capable of being applied to high torque has been required. Therefore, the method of simply increasing the diameter of the steering rack bar in the related art cannot be applied.
Additionally, in the related art, in order to achieve high strength of the steering rack bar, high frequency heat-treatment is performed to secure strength. However, if heat-treatment is performed before the cutting process, due to high strengthening of the material, it is difficult to perform the processing, and a thermal strain of the material occurs, and thus additional calibration is required. Accordingly, a production time increases, and thus production efficiency decreases and production costs increase.